"Some 20 migrants, mainly women and children, have entered Croatia near Tovarnik," close to the Serbian border on Wednesday, a police spokesman told AFP news agency.
At least 100 were seen walking through fields across the border, while 80 people crossed into Croatia after travelling by bus to the Serbian border town of Sid.
Refugees hope to reach Germany, as bitter debates continue in Europe over how to handle the crisis.
In August more than 156,000 people entered the European Union compared with 280,000 for the entire year of 2014.
In total, more than half a million have entered the EU in 2015 so far.
